Gas-discharge lamps are often used in lighting applications requiring a very bright light source. One example is a front lighting application, such as in a front headlight of a vehicle. Another example might be the illumination of an interior space such as an underground tunnel. A gas-discharge lamp for such applications is generally driven using AC (alternating current). In a front headlight application using a gas-discharge lamp as light source, a lighting module generally comprises a housing containing a burner and a driver. The term ‘burner’ includes a discharge vessel, usually of quartz glass and enclosing a fill comprising various metal salts, and an outer vessel that is also usually made of glass. The purpose of the driver is to regulate the lamp current and lamp power. For example, the driver can adjust the frequency and amplitude of the current as well as the level of the lamp power. To this end, a state-of-the-art driver usually comprises various electrical and electronic components such as semiconductor components for performing memory functions, logic functions, etc.
A gas-discharge lamp such as an automotive D5 lamp can easily operate for many thousands of hours under normal operating or environmental conditions. However, under certain circumstances, the temperature in the housing of the lamp may reach extreme levels, and the components of the driver, particularly temperature-sensitive semiconductor components, may not be able to withstand these temperatures. As a result, one or more driver components may become damaged and may even fail, so that the lifetime of the driver (and therefore the lifetime of the lamp itself) is significantly shortened.
One way of dealing with this problem might be to simply arrange the driver at a distance away from the lamp so that it is further away from the high temperatures that originate in the discharge arc and propagate through the electrodes. Alternatively, one or more large heat-sinks could be incorporated in the lamp design. However, in present-day automotive applications at least, a trend towards more compact headlight units means that the housing must also be quite compact. In such a design, the lamp driver must be located in close proximity to the burner. Such a compact design also cannot accommodate a large heat-sink.
In another approach, the lamp power could be reduced in order to also indirectly reduce the thermal load on the electronic components. However, reducing the lamp power, i.e. ‘dimming’ the lamp, has the direct consequence of lowering the temperature in the coldest spot of the discharge vessel. The term ‘coldest spot’ is used in its established context, namely to refer to the region in the discharge vessel that is coolest during operation. The coldest spot temperature should be kept as high as possible in order to achieve a desirably high efficacy. When the coldest spot temperature is lowered, the metal salts of the fill can partially condense and are subsequently unavailable in the gas phase, reducing the efficacy of the lamp, wherein efficacy is expressed as a ratio of the luminous flux to the power required to produce that luminous flux, i.e. lumens per Watt. The result is a noticeable drop in light output.
When the lamp power of an AC-driven lamp is reduced to approach a certain minimum, the commutation behaviour of the lamp can start to exhibit unfavourable behaviour. For example, at a zero-crossing of the lamp current, this may remain at or close to zero for a significant duration, so that the discharge arc becomes unstable. This is visible to an observer as a ‘flickering’ as the light output of the lamp fluctuates. If the lamp power is held at this minimum for too long, the discharge arc will most likely eventually extinguish.
Therefore, it is an object of the invention to provide a way of driving a gas-discharge lamp that avoids the problems described above.